What can Celtics expect from JaJuan Johnson?

If, like me, your first thought after the Celtics drafted JaJuan Johnson was, “Well, at least they got someone who could contribute right away,” then think again.

The C’s picked the 6-foot-10, 220-pound four-year collegiate power forward with the No. 25 pick in the first round of the 2011 NBA draft. In the five previous drafts, ever since the NBA banned high school players from joining the league until age 19 or one year after the graduation of their high school class, exactly 30 four-year college players have been drafted in the first round.

Of those 30 players, seven were either power forwards or centers like Johnson.

Of those seven players, four lasted past the lottery.

Of those four players, three played more than 10 games.

Of those three players, two averaged more than three points as rookies.

Of those two players, one measured under 7 feet, 2 inches tall: Trevor Booker.

It could be better (Roy Hibbert) or worse (DeMarre Carroll) than the 6-foot-7, 240-pound Booker, the No. 23 pick by the Wizards (via the Timberwolves) in 2010. To get an idea of what the Celtics should expect from their first-round pick if and when the 2011-12 season begins — and beyond — here’s how the seven other four-year collegiate big men have fared since 2006 …

Traded: Played for five different teams (Hornets, Kings, Rockets, Wizards and Hawks) in five NBA seasons

Playoffs: Averaged 2.1 points and 1.9 rebounds in 8.0 minutes during 20 playoff games in three of past four seasons

Not exactly a murderer’s row of NBA diamonds in the rough. Regardless of what kind of production the Celtics get out of Johnson, one thing is for sure: The Purdue product will be the only one of the seven four-year collegiate big men selected in the first round since 2006 to perform for a playoff team. How much he’ll contribute to that is highly suspect.